Switched power supplies of the type used in personal computers, fax machines, laser printers and the like, include a rectifier circuit which charges a capacitor from the line voltage. In most equipment, the initial charging current is limited to a nondestructive value by a series-connected, negative-temperature-coefficient thermistor. When power is applied, the thermistor is in a high-resistance state. When the rectifier draws current, the thermistor heats up and its resistance reduces to a very low value. The thermistor stabilizes in the low resistance state when at an elevated temperature.
It has been found that such protective devices may not be effective during power system transients resulting from a fault due to lightning or to physical damage to the transmission system. Such transients are typically characterized by rapid on/off cycling brought about by an automatically reclosing circuit breaker attempting to reclose itself after opening in response to the fault.
A problem associated particularly with computers is that the computer hard disk drive will not have time to come to a complete stop if power is restored very quickly after a shutdown. It is desirable for the drive to come to a complete stop before power is restored in order to avoid damage to the drive's heads.
Heretofore, there have been known circuits directed to protecting computers and other electronic equipment from problems resulting from a power interruption. Those circuits, however, have left much to be desired. For example, one such circuit does not actually disconnect the computer from the line voltage. Consequently, it cannot prevent damage to the computer's power supply when A.C. power is restored rapidly after an interruption. Another such circuit turns off the computer, or other equipment, very rapidly, for example, within one power cycle. Such rapidity results in unnecessary shutdowns when the power interruption is only momentary. Moreover, none of the known circuits automatically reconnects the electronic equipment to the power line when power is restored after a shutdown.